1. Field of the Invention
The invention is concerned with the fabrication of films of primary interest for use as masks or resists in the fabrication of printed circuitry.
2. Description of the Prior Art
Continuing development of printed circuits and integrated circuits has put increasingly stringent demands on fabrication techniques. Miniaturization combined with greater circuit complexity necessitating many sequential fabrication stages has resulted in a significant decrease in yield.
A particular problem involves the need for accurate registration of masks with patterns produced at intermediate stages in the fabrication of a circuit. An approach which has met some degree of acceptance involves a "see through" mask which may be optically aligned. This approach, implying a degree of transparency in the visible spectrum, as well as a required opacity for some energy used to define the pattern corresponding with the mask, is apparently satisfied by a film material which largely consists of one or more oxidized compounds of iron. See, for example, 118, Journal of the Electrochemical Society, 341 (1971).
Properly prepared films of such material are sufficiently transparent at 5890 Angstrom units for viewing purposes and are sufficiently opaque in the spectral region of from 3600-4000 Angstrom units for use as photomasks with an argon-ion laser. Such material is soluble in a number of acidic solutions--e.g., 6N HCl--with complete dissolution occurring within a time on the order of less than one hour at room temperature for film thicknesses ordinarily utilized.
The prior art utilizing iron oxide masks has generally made use of photolithographic techniques for pattern delineation. In accordance with this approach, photoresist coatings are exposed and developed in the usual manner following which bared iron oxide is etched away.
It is also known that demand for higher resolution may not be satisfied by use of photoresists and consideration has been given to other techniques which do not have the resolution limitations of photoresists. There are, for example, mask materials in which patterns may be generated by direct light beam "writing", for example, by use of coherent emission, particularly at higher frequencies. There has also been considerable effort directed to pattern generation by electron beam lithography. Apparatus is available for programming a scanning beam so as to "write" patterns directly in a variety of susceptible media. See, for example, IEEE Transactions on Electron Devices, ED-19, No. 5, page 624 (1972).
While iron oxide blanks (supported films on substrates of appropriate transparency intended for mask fabrication) are commercially available, it does not follow that all production problems have been solved. The technique which has gained commercial acceptance involves chemical vapor deposition (CVD) from iron pentacarbonyl. Use of an oxidizing atmosphere at temperatures within a specified range (sufficiently high to cause decomposition but insufficient to result in substantial insolubilization) results in film of adequate thickness, uniformity, appropriate optical property, and sufficient solubility to satisfy most purposes.
An objection to the CVD process is the attendant release of carcinogenic material into the atmosphere. It is characterized as well by other difficulties common to CVD processing. Alternative procedures involving sputtering, vaporization, and salt decomposition, are not known to be in commercial use.